Conventionally, a method using a partially saponified polyvinyl alcohol (polyvinyl alcohol may hereinafter be abbreviated as PVA) as a dispersion stabilizer for suspension polymerization of a vinyl compound (such as vinyl chloride) has been known. However, the partially saponified PVA has low solubility in water, and it is difficult to increase the solid content ratio, resulting in insufficient handleability. Therefore, in order to improve the handleability, attempts to use an aqueous emulsion as a dispersion stabilizer for suspension polymerization have been made. Further, in the suspension polymerization of a vinyl compound, attempts to control the properties of the resultant vinyl resin by the combination of PVA to be used are sometimes made. However, in the case of using the partially saponified PVA, the polymerization is rendered unstable depending on the combination of PVA to be used in the suspension polymerization, which may result in production of coarse particles or deterioration in properties of the vinyl resin to be obtained in some cases.
Examples of such an attempt to use an aqueous emulsion as the dispersion stabilizer for suspension polymerization of a vinyl compound include methods disclosed in Patent Documents 1 and 2. Examples of the attempts to devise a combination of PVA to be used include a method disclosed in Patent Document 3. In recent years, examples of requirements for the combination of a dispersant and a dispersion stabilizer to be used include (1) allowing high plasticizer absorption and easy processing, (2) allowing a small number of fish-eyes in sheet formation, (3) allowing excellent polymerization stability and a small number of coarse particles, (4) allowing a resin having high bulk specific gravity to be obtained, and (5) allowing excellent handleability of the dispersion stabilizer.
According to Patent Document 3, two types of PVA having different degrees of saponification and degrees of polymerization are used together with a partially saponified PVA. The partially saponified PVA is not dissolved or dispersed in water, thus having a problem in handleability mentioned above as the requirement (5), and the other performance also cannot be said to be satisfactory.